At present the most widespread raw material for the production of quartz glass and quartz ceramics is grit of naturally-occurring .alpha.-quartz, produced by processing rock crystal or vein quartz. This processing comprises crushing of naturally-occurring quartz, screening thereof; magnetic separation of the fraction of 0.1-0.5 mm, followed by flotation, chemical enrichment by treating with mineral acids, washing and drying.
A great number of complicated operations of processing of naturally-occurring quartz, use of sophisticated equipment and harmful operation conditions for the personnel are the basic disadvantages characteristic for the use of naturally-occurring quartz.
High-quality quartz glass is produced from grit of synthetic single crystals of quartz.
Known in the art is a process for producing single crystals of quartz by hydrothermal synthesis (cf. V. S. Balitsky, E. E. Lisitsyna "Sinthetic Analogs and Imitations of Naturally Occurring Precious Stones", Moscow, "Nedra" Publishing House, 1981, pp. 80-81; K.-T. Wilke "Methods of Growing Crystals", Leningrad, "Nedra" Publishing House, 1968, pp. 117). Quartz single crystals are obtained by way of recrystallization of naturally-occurring vein quartz in soda or alkali solutions at a temperature of 350.degree. to 420.degree. C. under a pressure of from 500 to 1,500 atm. High-quality single crystals weighing up to 2 kg are grown in industrial autoclaves for 3 to 12 months. Then these crystals are subjected to crushing enrichment, drying and screening. For the production of quartz glass the fraction of 0.1-0.5 mm particle size is used. About 2/3 of the raw material amount is not used in the production of the glass. The process is economically inefficient due to high rates of power and raw material consumption.
Known in the art is a process for producing polymorphous silica by heating amorphous silica in air in the presence of oxides of alkali metals at a temperature within the range of from 400.degree. to 1,000.degree. C. (cf. FRG Pat. Nos. 2,743,143 and 2,751,443 Cl. C 01 B 33/12). In this process only partial crystallization of silica occurs simultaneously with the formation of various modifications of crystalline silica with different melting points and different density values, as well as with a high content of water. All this hinders the use of the obtained product for the production of quartz glass.
Known in the art is a process for producing cristobalite which can be used as raw material for fusing-on quartz glass. Cristobalite can be obtained by way of high-temperature treatment of .alpha.-quartz (cf. U.S. Pat. No. 3,565,595 Cl. 65-21; GDR Pat. No. 134,219 Cl. C 03 B 1/00), as well as amorphous silica in the presence of mineralizers (cf. USSR Inventor's Certificate No. 600094, Cl. C 03 B 1/00; USSR Inventor's Certificate No. 776994, Cl. C 03 C 3/06). The process is economically less efficient and difficult to implement due to the necessity of lasting (up to 20 hours) heating at high temperatures (1,300.degree.-1,700.degree. C.). The use of mineralizers contaminates the glass produced from cristobalite. The process comprises a great number of operations. Thus, in the case of using amorphous silica as a raw material, the process consists in the following steps: amorphous silica is soaked in a solution of a mineralizer-alkali, ammonium fluoride or soda, briquetted and subjected to heat-treatment at a temperature of from 1,300.degree. to 1,700.degree. C.; the resulting sintered aggregates of cristobalite are crushed and screened to give a fraction of 0.1-0.5 mm particle size. Frequently amorphous silica is moulded, e.g. into tubes, prior to the heat-treatment, using aqueous solutions of binding agents such as polyvinyl alcohol (cf. Chem.Abstr. 1973, vol. 79, No. 16, p. 265, Abstr. No 96136d; Japan Kokai 73 36, 214, Cl. 21Cl; Chem.Abstr., 1976, vol. 85, No. 14, p. 268, Abstr. No. 98381c, Japan Kokai 76 77,612, Cl. C 03 B 3/06), potassium silicate with formamide (cf. U.S. Pat. No. 4,059,658; Cl. C 04 B 35/14), silicon halide (cf. British Pat. No. 1,168,934 Cl. C03C 1/02), and the like.
The most advantageous is the use, in the production of quartz glass, of a fine-crystalline quartz with a particle size distribution meeting the requirements of modern fusing-on methods.
Known in the art is a process for producing fine-crystalline .alpha.-quartz with a crystal size below 0.2 mm from amorphous silica by hydrothermal treatment of solid-phase mixtures of silica and ammonium fluoride in a weight ratio of from 0.5:1 to 5:1 respectively in an autoclave at a temperature of 150.degree.-400.degree. C. under a pressure of 5-15 atm (cf. USSR Inventor's Certificate No. 489723, Cl. C 03 C 3/06).
The process is economically inefficient, since it does not enable 100% crystallization of amorphous silica, necessitates lining of the autoclave with platinum or any other noble metal (due to the aggressive character of the medium); furthermore, the amount of amorphous silica charged into the autoclave is relatively small. The process also features harmful conditions of labour for the operating personnel. The resulting fine-crystalline .alpha.-quartz contains a great number of volatile impurities (fluorine, ammonia, water, and the like) thus making it impossible to produce a high-quality quartz glass therefrom.
Known is a process for growing fine-crystalline .alpha.-quartz from amorphous silica dissolved in an aqueous solution of ammonium fluoride in an autoclave at a temperature of 100.degree.-450.degree. C. under a pressure of 100-1,000 atm using needs of fine-crystalline .alpha.-quartz with a crystal size of 0.001-0.1 mm (cf. USSR Inventor's Certificate No. 491593, Cl. C 03 C 3/06). This prior art process features the same disadvantages as the one described hereinabove.